From what Reini tells me, he is mostly focused on making some fundamental changes to the Perl 5 language (unlikely to be adopted)

and

Reini is the only one working with ALL of Perl 5, not just a subset of Perl 5.

*Bzzzt!* Does not compute! Does not compute!

You can't be working for all of perl if, in the meantime, you're changing the language.

So, what parts of Perl would he be changing, syntax-wise or semantics-wise?

If you're trying to compile all of perl, then you may just have to rewrite the parts that it would like to handle differently. That rewrite should be done by the compiler or a precompiler, not by a programmer.